Photosynthetic/Photorespiratory Carbon Metabolism in the C3-C4 Intermediate Species, Moricandia arvensis and Panicum milioides

The distribution of (14)C in photosynthetic metabolites of two naturally occurring higher plants with reduced photorespiration, Moricandia arvensis and Panicum milioides, in pulse and pulse-chase (14)CO(2) incorporation experiments was similar to that for the C(3) species, M. foetida and Glycine max. After 6 seconds of (14)CO(2) incorporation, only about 6% of the total (14)C fixed was in malate and aspartate in both M. arvensis and P. milioides. The apparent turnover of the C(4) acids was very slow, and malate accumulated during the day in M. arvensis. Thus, C(4) acid metabolism by M. arvensis and P. milioides had no significant role in photosynthetic carbon assimilation under the conditions of our experiments (310 microliters CO(2) per liter, 21% O(2), 1100 or 1900 micromoles photon per square meter per second, 27 degrees C).After a 36-second chase period in air containing 270 microliters CO(2) per liter, about 20% of the total (14)C fixed was in glycine with M. arvensis, as compared to 15% with M. foetida, 14% with P. milioides, and 9% with G. max. After a 36-second chase period in 100 microliters CO(2) per liter, the percentage in glycine was about twice that at 270 microliters CO(2) per liter in the C(3) species and P. milioides, but only 20% more (14)C was in glycine in M. arvensis. These data suggest that either the photorespiratory glycine pool in M. arvensis is larger than in the other species examined or the apparent turnover rate of glycine and the flow of carbon into glycine during photorespiration are less in M. arvensis. An unusual glycine metabolism in M. arvensis may be linked to the mechanism of photorespiratory reduction in this crucifer.